Simple Lie Detector

Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.

Schematic

Parts

Part

Total Qty.

Description

Substitutions

R1

1

33K 1/4W Resistor

R2

1

5K Pot

R3

1

1.5K 1/4W Resistor

C1

1

1uF 16V Electrolytic Capacitor

Q1

1

2N3565 NPN Transistor

M1

1

0-1 mA Analog Meter

MISC

1

Case, Wire, Electrodes (See Nots)

Notes

The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.

To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.

Comments

To the person asking that a "how to build this" video be sent to him: (prince-something was username)...
Dude, if you cant build THIS circuit, (which by the schematic you could almost thumb-twist together) maybe you should consider NOT going anywhere NEAR ELECTRONICS, OR ANYWHERE you could hurt yourself or others... In other words, ask George to tell you about the rabbits..! And dont leave home without wearing your helmet!!

Dear Sir or Ma'am, I would like to ask if the pointer of the analog meter goes down when somebody lies or is my analog meter the problem? Also, when it is being tested, does the meter have to move from 0 then so on or from any number as long as it moves then it is lying? please reply asap. i need thisproject two days from now :(

i am a BTECH 3rd year student .i want to submit this project in my college as a minor project so please give me it's complete information project report and pdf file .please send as early as possible we have a limited time.thank u ..................

I am a BTECH 3rd year student . This time i m in a govt. company for training & i really impressed by this project so i decide to make it. I want to submit ths project in my college as a minor project . So plzzz give me it's complete information , project report and pdf file. Thanking you

How this circuit works: This ciruit works by detecting changes in a person's conductance. Our body always conducts (n) degree electricity, and when there is moisture (sweat in this case) -- this degree of condcutance increases. When a person is lying, their body produces sweat, and this circuit picks up on this. This circuit could also double as a liquid detector too.

Can you put step by step directions on how to build this or can you send it to me in an email because i am building a lie detector test for science fair and i don't understand this picture (p.s. i'm only in 8th grade). So can you please do me a favor and simplify this? Thank You

Hi, Is at all possible to incorporate a green and/or red LED in the circuit? So say you ask a question and its a lie the LED lights up. I am new to electronics so where would I place the LED? Thanks Mike H.

Why go through the trouble of making a lie detector? Their are plenty of resources out their to help you catch liars by certain techniques. Check out
www.humanliedetection.com it has a bunch of different techniques you can use. It is a much better alternative then actually hooking someone up to a lie detector.

hey im doin a lil project for my electronics program. i gota take a circuit off the net and make a bussiness proposal. i like the idea, jsut wondering if you could give me some better specs and details. im new to this..anything would be appreciated thank you

I've noticed that a lot of students want to do this for a project, but they do not seem to have even the basic knowledge of how circuits are built or even the components involved. I wanted to do this for a project, but it only contains one transistor and I needed at least three. So, I went with a Schmitt Trigger Oscillator. My prof. was right -- I am at the best school to learn electronics.....lol

I would like to repeat what the editor said: any small npn transistor will do. I tried 2N2222 and it worked fine. It worked, but i used a 0-100mA analog meter so it's difficult to see the change when someone lies. I'm from the Philippines and the whole circuit cost me more or less Php100 (it would have cost less if I searched the my whole city). One more thing: the lie detector only works when someone feels the NEED to lie, like when someone's reputation or life or something very important is at stake. I would like to send my kudos to the genius who made this. This is my first electronics project and was just done making it.

The three components of a lie detector test are perspiration, heart rate, and breathing. This contraption along with a telemetric band (like the one you buy for jogging at a sporting goods store... as seen in the new Incredible Hulk) would address two out of the three factors. Remember also that it is critical for your subject to believe in the accuracy of the test. There are some techniques available online in respects to that. Enjoy!!

of course it works! by sensing skin resistance what your really sensing is how much your victim/subject is sweating something liars do often. skin conductivity is actually one of the things professional lie detector units measure. along with heartrate, breathrate, etc.

I just build it today, and it worked gr8...I did however change a couple of things. Instead of the analog meter, i used an LED which lit on when the person was lying. When u make the electrodes touch each other they should light up. I also used a 330 ohm resistor instead of the 1.5kohm resistor in order to make the LED brighter. Also, make sure that the person's hand is super dry before they are tested. I don't believe you can get shocked from it, every1 that tried it out 2day had no shocks..i mean it's only 5v.

Studied the electronics for years and i see that provided the skins resistance drops then the circuit could have worked... It can easily been fooled... Ohh and smart move placing an analog 0-1mA meter there i've seen a few placing a pc speaker.. however it is not easy to find such an instrument everywhere...anyway good job there

hey i have a little question: i can't find any 0-1 mA Analog Meter, could i do any modification an use a 0-100mA analog meter and if that is possible can you what spares i should buy and th way i should connect them?

Hello...Tanks for the simple model. The principle involved is changing resistance thats nice, I am looking for an cicruit that works with the changing heart beats...please let me know if you have some thing like that....

I build it but it seems that it takes a lot of stress to make the potentiometer to react. Is there something that could be done to test it? Like inserting the electrodes in some substance or something w/o damaging the circuit?

I tried to use this on my wife and I electricuted her!
My lawyer says I have to cite your notes in court later.
Thanks a heap!
P.S. Turns out she may have been telling the truth about drinking all the milk.

Is that our skin resistance changes only when we lie? even our other expressions may cause the same isn't it! then how could we say the other person is lyeing,is that because his skin resistance has changed? Please clarify my doubts....its very interesting

Gina: It would still work, the wattage doesn't change anything here.
All the others: You should read something about electronics, understand how they work, how to build a simple circuit, how to recognize the components (Resistors, Capacitors, Transistors, LEDs, and all the others). After you do that you'll be able to build this...

Thank you for the project help .... this has been a very usefull tool on helping me move forward to the completion of my project though I greatly appriciate the idea i would like to inquire to if you could display a picture of the end reult of the "build it yourself" polygraph. Thank you.

hey..its gr8...but can u plz send me the steps of how to build one!...because in our experiment we have to BUILD our own..so PLZZ .send me the steps as soon as possible..i would really appreciate it..thx alot

hey, i'm making this lie detector for my science fair project this year..i wanted to know if it was an easy thing to make..and if they were certain steps to follow before following the diagram...let me know as soon as possible..thanks madelaine

Hi, im doing this for my science fair and i printed out the page thinking that it would tell me how to make this and it doesn't so if you would plz send me the information on it that would be VERY helpful. plaese send it A.S.A.P pranav

PPL READ THIS. he is no longer answering to emails so there are no directions or anything so STOP asking him to send you stuff. i learned the hard way. there is no info to send, the pic is the directions. Sorry, to say no wait write hahaha

Hi, im doing this for my science fair and i printed out the page thinking that it would tell me how to make this and it doesn't so if you would plz send me the information on it that would be VERY helpful.
Please send A.S.A.P,
Caitlin

Please may I ask you to add a section in all your circuits which does the actual explaining as to how the circuit works. I mean what happens from input to output. I'm sure a lot of viewers would highly appreciate that as it would be detailed and educational. Kind Regards. JOHN M. DLAMINI.
e-mail:

This is really great. This would be really good for a science project testing which gender is a better liar. So it IS doing my own homework, because I'm not finding out HOW to make this, but WHO is a better liar. Awesome! I love this site!

My friend and i are entering a sci cair next year and we want to get a head start on it now, i was wondering if you could send me the "how to put together" part of this. This is if you will allow us to use this design, we will give all due respects to you. Please get back to me ASAP.
Truely
Lone Wolf

Hello! This project seems very interesting, and I would like to know more about it. Could you please send me a full analysis of this project, and step by step instructions? i mean the circuit desription and working,etc. i have to explain a ckt to my class by this week and i chose dis ckt so could you PLEASE giv me all the details as fast as possible Thank you so much!!

This is cute. I hope no one actually believes that this or any other machine can detect a person's lie. For all you students, build the circuit and perform some double blind studies and test the theory that skin resistance changes as a person lies. Then goto https://antipolygraph.org

i need the full description of the circuit and its working etc
i've got to explain a circuit to my class by the end of this week and i've chosen this circuit of urs so PLEASE giv me d details as fast as possible PLEEEEAAASE.....

Hello! This project seems very interesting, and I would like to know more about it. Could you please send me a full analysis of this project, and step by step instructions?
i mean the circuit desription and working,etc.
i have to explain a ckt to my class by this week and i chose dis ckt so could you PLEASE giv me all the details as fast as possible
Thank you so much!!

I love the idea of the experiment! I am doing a science fair "Crime" project, YEAR 2, and I am undergoing all the fields of forensics investigation. Can you please send me more information, such as step by step instructions on building this lie detector. Thank you very much!

hi i just wanted to have the details to this project, like how i would put all these things together, where to get them, etc. i really want to use this as a project so if you can please
e-mail me back i would really appriciate it!!!!

I have a thought I want to share with you. I would like to see someone like you get a grant or backing from some coporation so a high quality personal hand held lie detector could be developed. Hopefuly this device would be affordable, just think of the demand for this product. I have no doubt it would change the world as we know. I think everyone in the world would agree honesty has left the planet. Let's bring it back.

hello sir,i m d student of b.tech final year.we wanna make dis project as our engineering project.we actually want deep knowledge about dis.for ex-will it be really able to catch wen a person is lying becoz some times swatting is not happen much wen a person is lying.so at that time if we r not able to catch the lier?

This project seems interesting, a bit much for my simple minded self, but alas I'll give it a shot. Thanks for keeping this site up, helped a lot in picking a project, but I think I'll add and change a few details to make it even more interesting.
--Thanks.--

can u pls send me d details of this circuit m thinking of doing this as my final year projects.pls include tis working n all...pls mail me.and . can you please send more details regarding the circuit layout on the pcb,cost for making , nmore about its working and about analog meter.

For those that ask how to put this togeather, Purchase breadboard, this is non printed circuit board that you will be able to soldier the parts to, Solder the parts as shown in the diagram, space the components out and use small pieces of wire to connect them, each component should have two pieces of wire (prongs) sticking from it. insert the prongs through a hole and soldier a prong to an end of wire then soldier the wire to the next components prong. (when the two prongs of any component connect in any way they become a completed circuit so do not let them touch and do not soldier them togeather.) when you have connected each one to the next component then close the circuit by connecting the last part to the first.

im taking a lie detector test shortly and always have suffered from sweaty hands for no apparent reason iv been docs about it yet told there nothing to worry about would this be a prob in lie detection case as im telling the truth

I really didnt believe that there are such BIG FOOLS out there who actualy think that this 'lie detector' is gonna find out if their boyfriends or girlfriends are cheating on them or whatever else!!! Come on people.. It works on the principle on basic conduction. If u sweat, its gonna cause a change in skin resistance which indicates that you are 'lying'. there is no gurantee that the person actually is.. it might never change for one thing.. and even if it does, it might just be that the weather is hotter than what it normally is!!!!!!!!!!!

We made the lie detector but it reacts to low resistence. Why not higher resistence and it doesn't seem to react to your skin very well. What could we do? please email me back asap i need it for a science festival

I would contest the term useful; the effectiveness of the "professional" lie detectors is contested. So I wouldn't use this to decide who's your friend/ex/etc.
Also make sure those input voltages are 4.5V and not mains; it's a lie detector not an electric chair....

Hi my name is Cassandra, 20 years old. Im a straight forword young lady that doesn't lie im starting to have a relationship with a 22 year old young man. i wanted to know if he lying to me because some of his ex girlfriend told me that he as a child and when i ask him he denied it his not telling the truth but i really want to get the truth out of him. Because he ask me alot of question about my personal life i tell him the truth how should i make him tell me the truth and he can't bearely look through my eyes he always look away. to me it seems that he not telling the truth. think i should take a trip to THE LIE DETECTOR TEST the only way i want to find out if his lying to me by trackin him when he called me so i could know if he lying to me PLEASE I NEED YOUR HELP OR AN ADVISED

i think my girlfriend cheated on me but she says she didnt and she swaers she didnt but i saw her sneaking out of sum lads house and i dont know 2 beleave her or not. so cud u plz plz tell me if this rly works cuz if it does i will make it and see

"hi my name is John and i have a question I have been to cort the reason because a young girl acussed me of doing something i didn't do and i am saying the truth about everything that happened that day but she lies in cort. I come from a very cristian family and my parents thought me not to lie so I always say the truth, Well my question is if its possible for me or her or both of us to have a lie detector test done to prove that I am inoccent.So I wanted to know if there is a way to prove that in cort and how much would it be so i can get a lie detector test in cort. Can you please let me know as soon as posible thank you so much."...I actually believe you didn't do whatever you are being accused of because I don't think you are smart enough! .If I were you, I'd learn how to spell and properly speak ASAP!

hi my name is John and i have a question
I have been to cort the reason because a young girl acussed me of doing something i didn't do and i am saying the truth about everything that happened that day but she lies in cort. I come from a very cristian family and my parents thought me not to lie so I always say the truth, Well my question is if its possible for me or her or both of us to have a lie detector test done to prove that I am inoccent.So I wanted to know if there is a way to prove that in cort and how much would it be so i can get a lie detector test in cort. Can you please let me know as soon as posible thank you so much.

My friends and i are doing a project. We are building a lie detector and need to know what size resistors and transistors we need. I would appreciate it if you could try your best to help us. If you can't, thanks for looking at this letter.
Sincerly, Curtis Crow

LOL! Honestly (if you'll pardon the pun) they don't work.
There has never been any scientific evidence that Lie Detectors of any kind are more than about 60% accurate - and that's when the subject is being thoroughly interrogated. 60% accuracy is no higher than an interrogation WITHOUT the aid of a polygraph.

How do you set the electrodes?
I tried to measure the skin resistance using high sensitive Ohm-meter
But I do not see any convincing changes.
Do you set the electrode at the finger tips or at the back of the subject's hand.
But the back of a hand is very high in resistance...almost non-conductive.

hiii...
i m student of IC engg. 6th sem..
i want to do this project as it seems very exciting and something new...
but the comments here show that it is very unpredictable....
can u plzzzzzzzz guide me about this project???
thanking u...

have a few questions that I hope you can answer:1-Does the transistor have tp be exactly the 2N3565 NPN Transistor or would any similar NPN do? 2-In your schematic you have 4.5V batteries drawn. I canot figure out how to incorporate these into the project-any andvise? 3- Do you have any more detailed explainations on how to make this work? Thank you

There was no problem making it. But I'm not sure about the sensitivity readings...Might need much better electrodes.
I get a a difference with wet fingers but not with any lies.
Any ideas?
And for you other guys with questions.
Parts can be found everywhere, most electronicsshops in town or online. Swedish company www.elfa.se has alot.
I don't know about the transistor but I used another NPN one.

I have a few questions that I hope you can answer:1-Does the transistor have tp be exactly the 2N3565 NPN Transistor or would any similar NPN do? 2-In your schematic you have 4.5V batteries drawn. I canot figure out how to incorporate these into the project-any andvise? 3- Do you have any more detailed explainations on how to make this work? Thank you

If you would like to leave a comment, you can do so using the form below. Comments can be things like suggestions, successes or failures, improvements, similar circuits, error reports, an answer to a previous commenter's question, or anything in between. However if you would like to ask a question it is suggested that you use the forum instead. The forum provides a more robust environment for discussion.

Important: If you post a technical question here, there is no guarantee it will be answered. All technical questions should be posted to the forum. The owner of this site generally does not answer questions in comments.

All comments are subject to approval before they are displayed here, so your comment will not appear immediately. This is done mainly to prevent spam and other abuse. If you wish to remain anonymous, simply enter "anonymous" for your name and/or email.

Name:

Email:

Subject:

Comments:

Anti Spam Code:

Please enter the exact text shown in the image above into the box below.
Browser cookies must be enabled to comment on circuits.